Lately I've been building it relatively early, in a city that has good hammers- frequently my cap. The reason for this is because I like to take advantage of the Ancestors Hall. I generally build my first expo as early as I can, and then have a big expansion push when Magnus gets his second promotion. Shortly after that I build the Govt. Plaza and then the Ancestors Hall. The AH gives another 50% to settlers on top of the card (I haven't tested this and don't know how the numbers work) so I'm able to build them fast, and I chop in more from other cities. The free builders go a long way towards getting the new cities off the ground. I'll mention though, it isn't worth delaying your expansion for this; the idea is to expand hard before and after the Ancestors Hall, not to wait for it. This is particularly potent with Rome, because each new city is ready to contribute with a builder and a monument as soon as you plant it. I don't clock super-fast finish times but this has been pretty reliable for me on Deity. If anyone has ideas on how to better use GP buildings I'm all ears.